If the customer does not have an email address on the invoice or paid receipt then there will not be an email address associated with the order.
You can easily see from your ZenSales dashboard if an orders does not have a customer email address.
If an email address was not imported then the words "Add Email" will be under the customer's name in red text.
Click "Add Email".
A notification box will appear where you can paste or type in the customer's email address.
Click "OK" and the email address will now be associated with both the order and the customer.